To accommodate imperfections in the manufacture of small electric motors, at least one of the two shaft bearings is normally a self-aligning bearing. This ensures that the rotor can rotate freely even when the two bearings are not co-axially aligned.
A self-aligning bearing such as this normally comprises a bearing member, a housing enclosing the bearing member and a resilient retainer which holds the bearing member in the housing while, at the same time, allowing the bearing member to tilt or swivel within the housing. The bearing member has a cylindrical inner surface, for receiving a rotatable shaft, and a single part-spherical surface-of-revolution forming the whole of the outer surface of the bearing member. Opposite ends of this outer surface are supported by a convergent end of the housing and by the resilient retainer.
Radial loading on the bearing members of self-aligning bearings for larger electric motors may be adequately supported by the convergent end of the housing and by a relatively robust resilient retainer. Moreover, axial clearances can be limited to such an extent that axial movement and, therefore, simultaneous radial movement are sufficiently restricted to prevent overstraining of the retainer. However, in self-aligning bearings for smaller electric motors, the retainers are much more flimsy and, although axial clearances can be somewhat reduced, they permit much greater movement relative to the size of the components and it is much more likely that the retainers will be damaged in such a way as to be incapable of the holding the bearing member in place. Small electric motors of up to 0.5 kW capacity, which can be handled manually, are particularly prone to this type of damage because they can be easily dropped in such a way as to be subjected to shock loading. It is therefore necessary to limit radial movement of the bearing member.
This can be done by providing the housing with a part-spherical seat which engages the part-spherical surface-of-revolution on the outer surface of the bearing member and which contains a circular region which defines a plane, perpendicular to the axis of the housing, which contains the centre of curvature of the part-spherical seat. However, formation of a part-spherical seat is relatively costly and this solution precludes the use of a simple frusto-conical seat. It is therefore more usual to provide a support ring which is supported within the housing and has an inner portion surrounding the bearing member with an annular clearance space between the support ring and the bearing member for limiting radial movement of the bearing member.
It is therefore known to provide a self-aligning bearing such as this in which a bearing member has an inner surface, for receiving a rotatable shaft, and an outer surface comprising a first external, part-spherical, surface-of-revolution, formed around the axis of the bearing member; a housing for accommodating the bearing member has a convergent end defining an internal surface-of-revolution engaging the first external, part-spherical, surface-of-revolution to provide the bearing member with both radial and axial support; a retainer is supported within the housing and has resilient means engaging the bearing member so as to urge the first external, part-spherical, surface-of-revolution axially into engagement with the convergent end of the housing; and a support ring has a radially inner portion surrounding the bearing member with an annular clearance space between the support ring and the bearing member for limiting radial movement of the bearing member.
A self-aligning bearing of this construction permits the bearing member, or any shaft supported within the bearing member, to be tilted or swivelled extensively, for example: through an angle of 30.degree., about an axis perpendicular to the axis of the bearing member. However, in use, it is only necessary for the bearing member to be capable of the tilting or swivelling through an angle of the order of 1.degree.. Moreover, there are situations in which it is disadvantageous if the bearing member is capable of tilting or swivelling movement permitted by known designs. Thus, in the assembly of small electric motors, particularly by automated processes, difficulty is encountered when the bearing member is excessively inclined to the axis of the housing because it is necessary to pass the rotor axially through the stator and, in this case, it is impossible to pass the shaft of the electric motor through the bearing member until the bearing member has been substantially aligned with the axis of the bearing housing and the remainder of the motor stator. Similarly, when the shaft of the electric motor has been inserted into the bearing member, it is often necessary to manipulate the part-assembled motor before the other end of the motor shaft is located in a second motor bearing. In this case, it is important to limit tilting or swivelling movement of the motor shaft so as to prevent the motor armature mounted on the shaft from fouling the motor stator. This also applies where it is necessary to transport the part-assembled motor before the second motor bearing is fitted.